The magnitude and dimensionality of spin–spin interactions among 2,2,6,6-tetramethyl-1-piperidinyloxyl radical (TEMPO) molecules in a 1D nanochannel consisting of tris(o-phenylenedioxy)cyclotriphosphazene (TPP) were examined using electron spin resonance (ESR) spectroscopy. Dilution of TEMPO with 2,2,6,6-tetramethyl-1-piperidine (TEMP), a diamagnetic molecule, reduced the dipolar interaction in the TPP/(TEMPO/TEMP) inclusion compound (IC), leading to axially symmetric hyperfine coupling and g-tensors with a molar fraction of 0–0.45: A⊥ = 1.93 ± 0.03 mT and A|| = 0.7 ± 0.1 mT, and g⊥ = 2.0063 ± 0.0003 and g|| = 2.0068 ± 0.0005. The resonance line in TPP/(TEMPO/TEMP) IC narrowed when the molar fraction was more than 0.7, implying the existence of exchange narrowing. The ESR lineshape of TPP/TEMPO IC showed pure 1D spin diffusion between 139 and 166 K, while interchain exchange interactions contributed to the line shape in T > 166 K. The magnitude of intra-chain exchange interaction, |Jintra|/kB, in TPP/TEMPO IC depended on the temperature from ≈0.05 K in 139–166 K to 0.4 K at 383 K. This aspect implies that molecular motion strongly affects the spin–spin interaction between TEMPO molecules in the TPP crystal. The magnetic susceptibility of TPP/TEMPO IC was also measured, and it was found that the exchange interaction was antiferromagnetic.